because with contraception, you have two people preform an act of self-giving, but hinder the natural purpose of their act, which is the conception of children, and which is the ultimate purpose for why God created sexuality, sexual union is a gift from God to the married, but by practicing contraception, married couples are accepting the pleasure God built into the act and yet denying Him its purpose, they are in effect mocking God, but remember that “God is not mocked.” (Gal. 6:7). Christ cursed the fig tree which, despite a fine external appearance, bore no fruit (Matt. 21:19; Mark 11:14). Marriage is God’s plan for populating Heaven, but those who use contraception refuse Him the specific fruit of their marriage, which is children, when they engage in the act which should produce children yet frustrate the natural, God-intended result…
and the Bishops do not say that, although if any do they are being disobedient to the church, but what Pope Pius XI has said about contraception is - “Since therefore, openly departing from the uninterrupted Christian tradition, some recently have judged it possible to declare another doctrine regarding this question, the Catholic Church, to whom God has entrusted the defense of the integrity and purity or morals…through our mouth proclaims anew: Any use whatsoever of matrimony exercised in such a way that the act is deliberately frustrated in its natural power to generate life is an offense against the law of God and of nature; and those who indulge in it are branded with the guilt of grave sin.”
if your grandmother won’t listen to reason or doctrine or even the popes, then all you can do is pray for her, although that’s what you must do first and foremost, before you even try to speak with her, because only God can really open her heart or mind to the truth, so never neglect to pray first,
and yet, if all your efforts fail at this time, just continue praying for her, especially the rosary, because you never know what great help in will afford her, either in this life or the next.
alright, hope this helps, take care.